SYS-CON MEDIA Authors: Liz McMillan, Elizabeth White, Yeshim Deniz, Roger Strukhoff, Jason Bloomberg

Related Topics: SYS-CON MEDIA

SYS-CON MEDIA: Article

Secrets Of The Masters: Core Java Job Interview Questions

JDJ Enterprise Editor Yakov Fain Offers 30 Core Java Questions You Might Expect During Job Interviews

JDJ's Enterprise Editor, Yakov Fain (pictured) writes: If you are planning to hit the job market,  you may need to refresh some of the Java basic terms and techniques to prepare yourself for a technical interview. Let me offer you some of the core Java questions that you might expect during the interviews. 

For  most questions  I’ve provided only  short  answers to encourage further research.  I have included only  questions for mid (*) and senior level (**) Java developers. These sample questions could also become handy for people who need to interview Java developers (see also the article "Interviewing Enterprise Java Developers").

Disclaimer. This article has been originally published three or four years ago, hundreds of thousands Java developers have read it, but I still use some of these questions while interviewing Java developers. Guess what? Every other Java developer doesn't now the answers to some of them.

30 Java Interview Questions

* Q1. How could Java classes direct program messages to the system console, but error messages, say to a file?

A. The class System has a variable out that represents the standard output, and the variable err that represents the standard error device. By default, they both point at the system console. This how the standard output could be re-directed:

Stream st = new Stream(new FileOutputStream("output.txt")); System.setErr(st); System.setOut(st);

* Q2. What's the difference between an interface and an abstract class?

A. An abstract class may contain code in method bodies, which is not allowed in an interface. With abstract classes, you have to inherit your class from it and Java does not allow multiple inheritance. On the other hand, you can implement multiple interfaces in your class.

* Q3. Why would you use a synchronized block vs. synchronized method?

A. Synchronized blocks place locks for shorter periods than synchronized methods.

* Q4. Explain the usage of the keyword transient?

A. This keyword indicates that the value of this member variable does not have to be serialized with the object. When the class will be de-serialized, this variable will be initialized with a default value of its data type (i.e. zero for integers).

* Q5. How can you force garbage collection?

A. You can't force GC, but could request it by calling System.gc(). JVM does not guarantee that GC will be started immediately.

* Q6. How do you know if an explicit object casting is needed?

A. If you assign a superclass object to a variable of a subclass's data type, you need to do explicit casting. For example:

Object a; Customer b; b = (Customer) a;

When you assign a subclass to a variable having a supeclass type, the casting is performed automatically.

* Q7. What's the difference between the methods sleep() and wait()

A. The code sleep(1000); puts thread aside for exactly one second. The code wait(1000), causes a wait of up to one second. A thread could stop waiting earlier if it receives the notify() or notifyAll() call. The method wait() is defined in the class Object and the method sleep() is defined in the class Thread.

* Q8. Can you write a Java class that could be used both as an applet as well as an application?

A. Yes. Add a main() method to the applet.

* Q9. What's the difference between constructors and other methods?

A. Constructors must have the same name as the class and can not return a value. They are only called once while regular methods could be called many times.

* Q10. Can you call one constructor from another if a class has multiple constructors

A. Yes. Use this() syntax.

* Q11. Explain the usage of Java packages.

A. This is a way to organize files when a project consists of multiple modules. It also helps resolve naming conflicts when different packages have classes with the same names. Packages access level also allows you to protect data from being used by the non-authorized classes.

* Q12. If a class is located in a package, what do you need to change in the OS environment to be able to use it?

A. You need to add a directory or a jar file that contains the package directories to the CLASSPATH environment variable. Let's say a class Employee belongs to a package com.xyz.hr; and is located in the file c:\dev\com\xyz\hr\Employee.java. In this case, you'd need to add c:\dev to the variable CLASSPATH. If this class contains the method main(), you could test it from a command prompt window as follows:

c:\>java com.xyz.hr.Employee

* Q13. What's the difference between J2SDK 1.5 and J2SDK 5.0?

A.There's no difference, Sun Microsystems just re-branded this version.

* Q14. What would you use to compare two String variables - the operator == or the method equals()?

A. I'd use the method equals() to compare the values of the Strings and the == to check if two variables point at the same instance of a String object.

* Q15. Does it matter in what order catch statements for FileNotFoundException and IOExceptipon are written?

A. Yes, it does. The FileNoFoundException is inherited from the IOException. Exception's subclasses have to be caught first.

* Q16. Can an inner class declared inside of a method access local variables of this method?

A. It's possible if these variables are final.

* Q17. What can go wrong if you replace && with & in the following code:

String a=null; if (a!=null && a.length()>10) {...}

A. A single ampersand here would lead to a NullPointerException.

* Q18. What's the main difference between a Vector and an ArrayList

A. Java Vector class is internally synchronized and ArrayList is not.

* Q19. When should the method invokeLater()be used?

A. This method is used to ensure that Swing components are updated through the event-dispatching thread.

* Q20. How can a subclass call a method or a constructor defined in a superclass?

A. Use the following syntax: super.myMethod(); To call a constructor of the superclass, just write super(); in the first line of the subclass's constructor.

More Stories By Yakov Fain

Yakov Fain is a Java Champion and a co-founder of the IT consultancy Farata Systems and the product company SuranceBay. He wrote a thousand blogs (http://yakovfain.com) and several books about software development. Yakov authored and co-authored such books as "Angular 2 Development with TypeScript", "Java 24-Hour Trainer", and "Enterprise Web Development". His Twitter tag is @yfain

Comments (176)

Share your thoughts on this story.

Add your comment
You must be signed in to add a comment. Sign-in | Register

In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.


Latest Stories
Tapping into blockchain revolution early enough translates into a substantial business competitiveness advantage. Codete comprehensively develops custom, blockchain-based business solutions, founded on the most advanced cryptographic innovations, and striking a balance point between complexity of the technologies used in quickly-changing stack building, business impact, and cost-effectiveness. Codete researches and provides business consultancy in the field of single most thrilling innovative te...
CloudEXPO has been the M&A capital for Cloud companies for more than a decade with memorable acquisition news stories which came out of CloudEXPO expo floor. DevOpsSUMMIT New York faculty member Greg Bledsoe shared his views on IBM's Red Hat acquisition live from NASDAQ floor. Acquisition news was announced during CloudEXPO New York which took place November 12-13, 2019 in New York City. Our Silicon Valley 2019 schedule will showcase 200 keynotes, sessions, general sessions, power panels, and...
Growing a digital business is essential for virtually every enterprise across every industry. Large enterprises and small businesses alike are digitizing their internal and customer-facing processes and services to increase efficiency, scale their businesses, and collect meaningful data. Yet, while many companies have launched digital services to collect data, we haven’t fully tapped into the potential of data analytics—even though studies suggest that digital will be a top differentiator in yea...
Darktrace is the world's leading AI company for cyber security. Created by mathematicians from the University of Cambridge, Darktrace's Enterprise Immune System is the first non-consumer application of machine learning to work at scale, across all network types, from physical, virtualized, and cloud, through to IoT and industrial control systems. Installed as a self-configuring cyber defense platform, Darktrace continuously learns what is ‘normal' for all devices and users, updating its understa...
ShieldX's CEO and Founder, Ratinder Ahuja, believes that traditional security solutions are not designed to be effective in the cloud. The role of Data Loss Prevention must evolve in order to combat the challenges of changing infrastructure associated with modernized cloud environments. Ratinder will call out the notion that security processes and controls must be equally dynamic and able to adapt for the cloud. Utilizing four key factors of automation, enterprises can remediate issues and impro...
As you know, enterprise IT conversation over the past year have often centered upon the open-source Kubernetes container orchestration system. In fact, Kubernetes has emerged as the key technology -- and even primary platform -- of cloud migrations for a wide variety of organizations. Kubernetes is critical to forward-looking enterprises that continue to push their IT infrastructures toward maximum functionality, scalability, and flexibility. As they do so, IT professionals are also embr...
At CloudEXPO Silicon Valley, June 24-26, 2019, Digital Transformation (DX) is a major focus with expanded DevOpsSUMMIT and FinTechEXPO programs within the DXWorldEXPO agenda. Successful transformation requires a laser focus on being data-driven and on using all the tools available that enable transformation if they plan to survive over the long term. A total of 88% of Fortune 500 companies from a generation ago are now out of business. Only 12% still survive. Similar percentages are found throug...
CloudEXPO has been the M&A capital for Cloud companies for more than a decade with memorable acquisition news stories which came out of CloudEXPO expo floor. DevOpsSUMMIT New York faculty member Greg Bledsoe shared his views on IBM's Red Hat acquisition live from NASDAQ floor. Acquisition news was announced during CloudEXPO New York which took place November 12-13, 2019 in New York City.
OpsRamp is an enterprise IT operation platform provided by US-based OpsRamp, Inc. It provides SaaS services through support for increasingly complex cloud and hybrid computing environments from system operation to service management. The OpsRamp platform is a SaaS-based, multi-tenant solution that enables enterprise IT organizations and cloud service providers like JBS the flexibility and control they need to manage and monitor today's hybrid, multi-cloud infrastructure, applications, and wor...
The Master of Science in Artificial Intelligence (MSAI) provides a comprehensive framework of theory and practice in the emerging field of AI. The program delivers the foundational knowledge needed to explore both key contextual areas and complex technical applications of AI systems. Curriculum incorporates elements of data science, robotics, and machine learning-enabling you to pursue a holistic and interdisciplinary course of study while preparing for a position in AI research, operations, ...
Codete accelerates their clients growth through technological expertise and experience. Codite team works with organizations to meet the challenges that digitalization presents. Their clients include digital start-ups as well as established enterprises in the IT industry. To stay competitive in a highly innovative IT industry, strong R&D departments and bold spin-off initiatives is a must. Codete Data Science and Software Architects teams help corporate clients to stay up to date with the mod...
Atmosera delivers modern cloud services that maximize the advantages of cloud-based infrastructures. Offering private, hybrid, and public cloud solutions, Atmosera works closely with customers to engineer, deploy, and operate cloud architectures with advanced services that deliver strategic business outcomes. Atmosera's expertise simplifies the process of cloud transformation and our 20+ years of experience managing complex IT environments provides our customers with the confidence and trust tha...
Industry after industry is under siege as companies embrace digital transformation (DX) to disrupt existing business models and disintermediate their competitor’s customer relationships. But what do we mean by “Digital Transformation”? The coupling of granular, real-time data (e.g., smartphones, connected devices, smart appliances, wearables, mobile commerce, video surveillance) with modern technologies (e.g., cloud native apps, big data architectures, hyper-converged technologies, artificial in...
With the introduction of IoT and Smart Living in every aspect of our lives, one question has become relevant: What are the security implications? To answer this, first we have to look and explore the security models of the technologies that IoT is founded upon. In his session at @ThingsExpo, Nevi Kaja, a Research Engineer at Ford Motor Company, discussed some of the security challenges of the IoT infrastructure and related how these aspects impact Smart Living. The material was delivered interac...
Intel is an American multinational corporation and technology company headquartered in Santa Clara, California, in the Silicon Valley. It is the world's second largest and second highest valued semiconductor chip maker based on revenue after being overtaken by Samsung, and is the inventor of the x86 series of microprocessors, the processors found in most personal computers (PCs). Intel supplies processors for computer system manufacturers such as Apple, Lenovo, HP, and Dell. Intel also manufactu...